This invention relates to metal sheet, and more specifically to a method for producing aluminum clad magnesium by twin roll casting.
Magnesium is approximately ⅔ of the density of aluminum and thus has advantages in weight-critical applications. Magnesium sheet has been used since the 1930's for a number of aerospace applications. During the second world war, both aluminum and magnesium sheet were widely utilized, but after the war, only aluminum successfully made the transition to the consumer market. Magnesium sheet was briefly used for some premium applications but began to fade out and virtually disappeared from the consumer market by the 1970's. Magnesium's demise was mainly due to the high cost of magnesium sheet. The high cost was a result of the high price of magnesium metal and the expensive transformation process required to convert the metal into sheet.
Immediately after the war, both aluminum and magnesium were manufactured in a similar manner. The metal was first cast into an ingot which was then cooled to room temperature. To remove casting imperfections, the top and bottom of the ingot were removed by sawing and the faces of the ingot were scalped. The ingot was then reheated and hot rolled to make a coil of 3 mm to 7 mm thick strip. In the case of magnesium, the ingot was sometimes substituted by an extruded slab.
In the case of aluminum, the hot rolling was initially performed on a reversing hot mill. The mill rolls the ingot into a plate that becomes progressively thinner and longer. The plate traverses either side of the mill on “run-out” tables. Depending on the desired coil size and production required, the final passes are either performed on this same reversing mill using coilers that are engaged on one, or both, sides of the mill, or alternatively, the rolled plate was passed into a series of tandem hot mills which rolled the plate in one continuous operation into the finished coil. The tandem mill approach had a higher production and could make larger coils. After leaving the hot mill, the coils could then be cold rolled at room temperature to the final gauge required for the finished product. Most modern, high volume, production processes require the sheet to be delivered in coil form. Hot mills use a water based emulsion as a lubricant and cold mills use a light oil similar to kerosene.
In the case of magnesium and most common magnesium sheet alloys, the metal due to its hexagonal close-packed crystal structure of the metal limits its deformation abilities at lower temperatures. This required frequent reheating in off-line ovens to maintain the temperature between 250 C and 450 C. Below this temperature the metal had the tendency to crack during rolling. Handling and reheating oven constraints limited the maximum slab size and traditionally made magnesium sheet production virtually a sheet-by-sheet operation. This was a very labor and energy intensive, inefficient method of production and contributed to the high cost of magnesium sheet. Even today, only small size coils of magnesium sheet are available.
The aluminum industry saw significant change in the 1950's when twin roll continuous casters were developed. This machine used twin water cooled rolls to simultaneously cast and warm roll molten aluminum directly into coils of 3 mm to 7 mm thick, a gauge suitable for subsequent cold rolling. This process eliminated the ingot casting and hot rolling operations and thus significantly reduced the conversion cost of transforming the aluminum to finish gauge. This lower transformation cost reduced the price of aluminum sheet and helped aluminum make the transition from an aerospace material to an everyday material used for products such as construction, transportation and packaging.
As stated above, magnesium, having a lower density than aluminum, has advantages in weight-critical applications. Two potential major markets for magnesium sheet include the consumer electronics industry such as cell phones, notebook computers, cameras, MP3 players, etc., and in automotive applications. The light weight, thermal and electrical conductivity, electromagnetic shielding and dent resistance of magnesium make it attractive for personal electronics, whereas the potential weight savings and resultant fuel efficiency make it attractive for automobiles. Magnesium alloys have previously been used in automobiles in the form of castings such as for instrument panel carriers, pedal brackets, seat components, etc., but the metal is difficult to use for applications such as engine blocks, transmission cases, etc., as it has poor elevated temperature creep properties. Thus to further increase the use of magnesium and magnesium alloys in automobiles, it must be available in sheet form.
Magnesium does, however, have some significant disadvantages for both electronic and automotive applications, including that magnesium is more reactive than aluminum. As magnesium must be finished rolled at an elevated temperature and because it must be formed into a finished part at an elevated temperature, the surface tends to be more oxidized and must be cleaned after forming which adds to cost. Considering magnesium is more reactive than aluminum, it has a lower corrosion resistance. This is particularly important for automobile applications. In corrosive environments, the magnesium sheet surface requires special finishing treatments that form a protective coating. These finishes are typically some form of anodic or conversion coating, but metallic plating has also been used for aerospace applications. Both types of coating are prone to micro pinholes that allow diffusion of the magnesium to the surface reducing the corrosion resistance of the coated sheet. In the case of metallic plating where an electrically conductive film of gold, aluminum or the like is applied to the surface, a galvanic cell is formed between the magnesium and the coating, thereby accelerating the corrosion of the magnesium. Furthermore, these protective coatings are thin and susceptible to damage that will also result in localized corrosion.
Although paint systems for aluminum autobody parts are well established, they need to be developed and optimized for magnesium sheet. Similarly, spot welding technologies for aluminum auto parts are well established, however they also need to be developed and optimized for magnesium sheet. Adhesive bonding systems for aluminum autobody parts are also well established, however they too need to be developed and optimized for magnesium sheet.
Considering there are corrosion problems when dissimilar metals are joined, one of the short term applications foreseen for magnesium sheet is to use it as the inner structure for hoods, trunk lids, doors, etc., and use aluminum for the outer panel. This design creates a potential for corrosion issues. Another problem is that there is a public perception that magnesium can easily catch fire. And finally, another disadvantage of magnesium is that magnesium has a lower elastic modulus than aluminum.
In recent years, the world magnesium market has seen an influx of inexpensive magnesium ingot manufactured abroad which has driven down magnesium metal prices and rekindled the interest in magnesium sheet. Although much of this magnesium price decline has been reversed in recent months, the increasing price of oil has maintained interest in the potential for energy saving by using light weight magnesium sheet. Consequently, a need exists to inexpensively produce magnesium sheet which considers and solves the problems inherent in the use of magnesium.